Well it is very possible. @ Scottsdale Gun Club, in the law enforcement corner, they have barrel mounted shotguns. I guess the easiest way to mount one is to either use some wire of some kind, glue or screw a rail clamp on the top of the shotgun and mount it like a M203 (like in the vid), or get a barrel mount and do the same thing. I also would suggest doing it on a gun with a full or crane stock so the weight of the battery balances out the gun, just like an M203.